Various sensors utilizing optical fibres are known. Many such sensors rely on fibre optic point sensors or discrete reflection sites such as fibre Bragg gratings or the like being arranged along the length of an optical fibre. The returns from the discrete point sensors or reflection sites can be analysed to provide an indication of the temperature, strain and/or vibration in the vicinity of the discrete sensors or reflection sites.
Fully distributed fibre optic sensors are also known in which the intrinsic scattering from a continuous length of optical fibre is used. Such sensors allow use of standard fibre optic cable without deliberately introduced reflection sites such fibre Bragg gratings or the like. The entire optical fibre from which a backscatter signal can be detected can be used as part of the sensor. Time division techniques are typically used to divide the signal returns into a number of time bins, with the returns in each time bin corresponding to a different portion of the optical fibre. Such fibre optic sensors are referred to as distributed fibre optic sensors as the sensor portions are fully distributed throughout the entire optical fibre. As used in this specification the term distributed fibre optic sensor will be taken to mean a sensor in which the optical fibre itself constitutes the sensor and which does not rely on the presence of specific point sensors or deliberately introduced reflection or interference sites, that is an intrinsic fibre optic sensor.
GB patent application publication No. 2,442,745 describes a distributed acoustic fibre optic sensor system wherein acoustic vibrations are sensed by launching a plurality of groups of pulse modulated electromagnetic waves into a standard optical fibre. The frequency of one pulse within a group differs from the frequency of another pulse in the group. The Rayleigh backscattering of light from intrinsic reflection sites within the fibre is sampled and demodulated at the frequency difference between the pulses in a group.
Distributed fibre optic sensing therefore provides useful and convenient sensing solutions that can monitor long lengths of optical fibre. Standard telecommunications optical fibre, e.g. single mode 125 μm optical fibre, can be used which means that the sensing fibre is relatively cheap and readily available and, in some instances, it may be possible to use existing optical fibres for acoustic monitoring.
The ability to use conventional fibre optic cable for distributed acoustic sensing is advantageous in a number of different applications. However for some applications the sensitivity of a DAS system on conventional fibre may not be sufficient, for instance for various seismic applications. Also in some applications it would be beneficial to be able to discriminate between stimuli incident from different directions.
WO2012/177547 describes a fibre optic cable in which directional sensitivity is enhanced. An inertial mass is attached to a fibre, and is able to move (possibly in a constrained manner) in a housing. However, incorporation of such an inertial mass into a sensor apparatus is complex to implement and places a strain on the fibre.